The long-term goal of this project is to understand the molecular basis of cilia formation and maintenance in vertebrate photoreceptor cells. In vertebrates, the assembly and maintenance of photoreceptor outer segments begins with the formation of a connecting cilium. The connecting cilium contains a microtubule- based axoneme that is anchored to the apical inner segment by a basal body. Cilia formation begins with the docking of basal bodies at the apical surface of the inner segment. Genetic mutations disrupting the assembly, structure, or function of basal bodies and/or cilia result in a spectrum of diseases known as ciliopathies. These multisyndromic disorders often present with retinal degeneration, kidney disease, mental retardation, and polydactyly. In the current application, we will utilize loss-of-function strategies in zebrafish to investigate the mechanisms controlling basal body localization. In Specific Aim 1, we will test the hypothesis that the dynein/dynactin complex regulates the apical transport of basal bodies preceding cilia formation by examining zebrafish mutants in dynein and the p150 and p50 subunits of dynactin. In Specific Aim 2, we provide preliminary evidence that basal bodies show a highly polarized arrangement within the adult zebrafish retina. We will directly test the hypothesis that the PCP pathway regulates this patterning and is essential for photoreceptor survival. In Specific Aim 3, we will examine zebrafish carrying null mutations in the Joubert Syndrome gene arl13b for retinal phenotypes. Proposed experiments will also test the requirement of the GTPase domain and a ciliary-targeting sequence RVxPx for Arl13b function. We will also test arl13b for functional interactions with Bardet-Biedl Syndrome (BBS) genes, and components of the PCP pathway. These interactions will identify potential second-site modifiers that enhance expression of photoreceptor phenotypes. The results of these studies will reveal novel mechanisms required for basal body placement prior to cilia formation and to identify novel genetic interactions that influence hereditary blindness.